1. Field of the Invention
The present invention relates to a pulse wave measuring apparatus for measuring changes in intra-arterial pressure by pressing a sensor against an artery with an appropriate pressure. In particular, the present invention relates to a pulse wave measuring apparatus controlling the internal pressure of an air bag used for pressurizing the artery, by supplying air into the air bag (increasing the pressure) or exhausting air from the air bag (reducing the pressure).
2. Description of the Background Art
Pressure wave generated as the heart beats and propagated through an artery or vibrations of the artery wall is/are generally called pulse wave. A pulse wave measuring apparatus has a sensor pressed against the surface of a measurement site of a subject's body in order to measure the pulse wave from an artery of the measurement site. For this purpose, the sensor has to be pressed against the subject's body with an appropriate pressure. When the sensor is pressed against the subject's body with an inappropriate pressure, the accuracy of the waveform of the pulse wave is deteriorated.
A pressing mechanism for pressing a sensor against a subject's body is disclosed for example in Japanese Patent Laying-Open No. 63-293424 which also discloses a pulse wave measuring apparatus including a pressure sensor for detecting the pulse wave, an air bag for pressing the pressure sensor against a subject's body, and a valve for adjusting the internal pressure of the air bag. The internal pressure of the air bag is adjusted by applying a drive signal to the valve.
A structure for adjusting the pressure by controlling supply of the air into the air bag or exhaustion of the air therefrom generally includes an air supply pump and a very-slow exhaust valve or quick exhaust valve that are employed in sphygmomanometers for example. The site of the subject's body to which the pressure is applied for measuring the pulse wave may be smaller in area than the site where the pressure is applied for measuring the blood pressure. Therefore, the air bag of the pulse wave measuring apparatus as well as the capacity thereof for keeping air are smaller than those of sphygmomanometers. It is thus extremely difficult to reduce, by minute amounts, the pressure in the air bag of the pulse wave measuring apparatus by exhausting the air at a very slow speed.
The sphygmomanometer measures the blood pressure by successively changing the internal pressure of the air bag, while the pulse wave measuring apparatus employs a control sequence as shown in FIG. 11A for applying the pressure. In FIG. 11A, the vertical axis represents the internal-pressure level of the air bag and the horizontal axis represents the passage of time. Further, FIG. 11B shows changes in output level of the pressure sensor as the internal pressure of the air bag is changed according to the sequence for applying the pressure. In FIG. 11B, the vertical axis represents the output level (pulse wave signal level) of the pressure sensor and the horizontal axis represents the passage of time corresponding to that shown in FIG. 11A.
Referring to FIG. 11A, pressurization is started at time T1, and the pressurization is continued until time T2 while searching for an optimum internal pressure level (pressurization level) which is a state herein called tonometry state. The optimum internal pressure level is an internal pressure level at which the amplitude of the waveform of the pulse wave obtained while the pressure is applied becomes constant.
In the period from the time when the pressurization is started to the time when the optimum internal pressure level is reached, the artery wall at the pressed site of the subject's body is curved by the pressing force, so that pressure applied (from the artery) to the pressure sensor increases due to influences of the tension of the curved artery wall. When the optimum internal pressure level is reached, the surface against which the sensor is pressed and the artery wall become almost in parallel with each other and accordingly, there is almost no influence of the tension of the artery wall on the vibrations of the artery perpendicular to the pressed surface. This state is the aforementioned tonometry state in which the pulse wave can accurately be detected.
At time T2, the internal pressure level is higher than the optimum internal pressure level. After time T2, the pressure is gradually reduced so that the determined optimum internal pressure level is reached. After time T3 at which the optimum internal pressure level is reached, the optimum internal pressure level is kept until the measurement is completed.
Referring to FIG. 11B, after the pressurization is started, the amplitude of the pulse wave signal gradually increases to become constant when the tonometry state is attained. As the pressurization is further continued, the bottom of the waveform of the pulse wave signal starts to distort. In the state where the optimum pressure level is maintained after time T3, the pulse wave is measured.
The control is effected by successively changing the internal pressure for determining the optimum internal pressure level and, after time T2 at which the internal pressure level exceeds the optimum internal pressure level, carrying out quick exhaustion (quick pressure reduction) and very-slow-speed exhaustion (very-slow-speed pressure reduction). Thus, in the short period between time T2 and T3, the pressure has to be reduced to the optimum internal pressure level and thereafter the optimum internal pressure level has to be maintained for a certain period of time until the measurement is completed. Accordingly, it has been a demand for a pressure control system having the functions of the quick exhaustion, very-slow-speed exhaustion and pressure maintenance.
Such a pressure control system is achieved for example by a syringe pump. The syringe pump is a device that finely controls the amount of air injected into a syringe which is a closed vessel or the amount of air discharged from the syringe by means of a stepping motor. A pulse wave measuring apparatus having the syringe pump mounted thereon cannot be reduced in size due to a large-sized control mechanism of the syringe pump and is not cost-effective because the syringe pump is expensive. In addition, when the pulse wave measuring apparatus having the syringe pump mounted thereon causes pain, due to pressurization, to a subject whose pulse wave is being measured, or when an emergency arises due to electric power failure for example, quick exhaustion of the syringe pump is difficult. Then, in order to address such a situation, a pump has to be added to the exterior for rapid exhaustion. It is seen from the above that, in order to mount the syringe pump on the pulse wave measuring apparatus and then satisfy functions required for measuring the pulse wave, various problems have to be solved.